1. Field of the Invention
This invention relates to a charging member and apparatus for providing electric charge to an object to be charged such as a photoconductor used in an electrophotographic or electrostatic recording process in copying and printing machines.
2. Prior Art
The conventional electrophotographic process used in copying machines is by evenly charging the photoconductor at its surface, projecting an image to the photoconductor from an optical system to eliminate electric charges in areas exposed to light, thereby forming a latent image, applying toner to the photoconductor to form a toner image, and transferring the toner image to paper. The first step of charging the photoconductor generally employs a corona discharge mode. However, the corona discharge mode is undesirable from the standpoint of safety and maintenance of the machine since application of voltage as high as 5 to 10 kV is required. The corona discharge also gives rise to an environmental problem since harmful substances such as ozone and NOx generate.
It was recently proposed to bring a charging member having voltage applied thereto in contact with an object to be charged, typically a photoconductor, thereby charging the object as disclosed in Japanese Patent Application Kokai (JP-A) Nos. 205180/1989 and 211779/1989. This contact mode enables charging at a lower applied voltage than the corona discharge mode and minimizes ozone evolution, indicating the possibility of overcoming the problems of the corona discharge mode.
Charging members used in the contact mode are rollers made of conductive rubber having conductive particles such as carbon dispersed therein with or without a coating of nylon or polyurethane. The conductive rubber rollers without a coating should have an increased loading of conductive particles for low resistivity, which in turn, results in an increased rubber hardness to damage the surface of an object to be charged. Chemicals blended in the rubber can migrate to the skin layer to contaminate the photoconductor to deteriorate its charging ability.
The conductive rubber rollers with a coating of nylon or polyurethane are effective for preventing contamination of the photoconductor, but experience changes in resistance with the surrounding environment. A low temperature, low humidity environment obstructs even charging, sometimes causing black peppers and fog upon reversal development. It is possible to control the resistance of the skin layer by adding conductive particles. However, since the resistivity largely varies with the amount of conductive particles added, the desired resistivity is available with difficulty from the standpoint of manufacture, often resulting in variations of charging ability.
More lately, it was found that the contact charging mode using conductive rubber rollers having conductive particles such as carbon dispersed therein raised some problems including noise upon application of AC voltage and a loss of the photoconductor's toner sticking inability due to direct contact.